Taking stock of hominid evolution

The dearth of fossils along humanity’s early evolutionary path inevitably results in even a single find forcing a rethink of the whole story.  Sometimes it exposes a novel characteristic, or a new date of occurrence, and quite minor deviations in relative durations of different species or minuscule differences in dentition or foot bones assume an importance that would be disproportionate in any other vertebrate group.  The last 2 to 3 years have unearthed evidence for the presence of bipedalism as early as 6 Ma ago, and three new primate divisions that seem on the line to humans rather than other living apes.  The 15 February issue of Science devotes 8 pages of News Focus to reviewing hominid evolution (Balter, M. and Gibbons, A. 2002.  Becoming human.  Science, v. 295, p. 1214-1225).

One picture that emerged more than a decade ago is that the richest pickings occur along the line of the East African Rift system, where continued extension since Miocene times has created room for the deposition of terrestrial sediments and thus chances of preservation.  Moreover, its continual volcanic activity has interleaved sedimentary strata with lava flows and ash beds that present ample opportunities for precise dating.  It is in the Rift that the onset of human-like traits has been pushed further and further back in time.  The discovery of Ardepithecus ramidus (“root Earth-ape) at Aramis in the Afar province of Ethiopia by The Middle Awash Research Team in 1992 (dated at around 4.4 Ma) pushed “Lucy” and the earlier, but fragmentary 4 Ma Australopithecus anamensis out of specialists’ ranking as the first in our line.  Last year Yohannes Haile Selassie published details of an earlier Ardepthicus subspecies from Afar, whose age is between 5.2 to 5.8 Ma.  In both, the central evidence for being hominid rests on foot bones, for the teeth bear a mixture of chimp- and human-like features.  Ardepithecines possibly could walk bipedally, but probably ate soft fruit and leaves in forested hills.  And then there is Orrorin tugenensis (“original man”) from the Tugen Hills in the Kenyan Rift, coming in at 5.72 to 5.88 Ma.  This so-called “Millennium Man”, found by a joint French-Kenyan team.  Its gait has still to rest on what to most of us might seem like flimsy evidence, modelled from three thighbones.  Orrorin’s teeth have mixed human- and chimp-like characters.  Unsurprisingly, Orrorin’s finders claim primacy as well as a nice new name, while those responsible for slightly younger Ardepithecus argue that both are the same genus.  The most important point, assuming that bipedality can be convincingly demonstrated for both, is that neither dwelt in grasslands, but in forests.  Bipedality might not have evolved through pressures that emerged with the spread of African savannah.  Although yet to be published, and dated only by stratigraphic means, an early forest dwelling hominid fossil, found last year in northern Chad by the French-Chadian Palaeoanthropological Mission breaks the stranglehold of the Rift on exploration for early hominids.  Two thousand kilometres from the Rift, the Chadian find implies that hominids roamed over a vast tract of a largely flat continental surface.

As well as a flurry of revisions to the human evolutionary “bush” (and each anthro to their own!), the oldest date comes dangerously close to the 5 to 7 Ma date of last common ancestor between the chimp and the human lines, as estimated from the difference between modern DNA sequences.  One among several possibilities is that the chimp human separation involved acceleration of evolution in our line; something often attributed to a “bottleneck” when numbers of individuals dropped to such a low level that mutations spread rapidly, instead of being “ironed out” in a larger gene pool.  There is one worrying aspect of the hunt for human ancestral fossils – there seems to be little parallel effort to seek early chimp fossils, or at least they are exceedingly rare.  That may be because true tropical rain forest with its highly oxidizing soils destroys the evidence.  Whatever, there is a possibility that among the increasing number of supposedly hominid fossils could be some ancestral chimpanzees!  All that would be required is a reversion to knuckle walking in forest environments.  Bone and tooth enamel cannot resolve that possibility.  The only possible way forward is more finds in a wider geographic diversity of sites, which the finds in Chad suggest is achievable, given Miocene to Pleistocene successions.

The thrust of research shifts from bones to artefacts in the case of Homo species, and how they might be interpreted in terms of cognitive ability.  Most important are signs of  abstraction from the natural world; in a word, art.  There has long been a Eurocentric bias, largely because of the wealth of exquisite objects that explode into the archaeological record there after 40 thousand years ago.  Art is a sure sign of fully human cognitive abilities, no matter how much physical anthropologists might ponder over this or that feature of skulls from the late-Pleistocene, and its role in shaping brain architecture and function.  Sudden European appearance of artistic expression has long spurred the view that its evolution was explosive and unique, probably as a result of some mutation.  That view needed revision as soon as Christopher Hinshilwood of the South African Museum reported his find in January this year of geometrically carved ochre objects close to Cape Town.  They are 77 thousand years old, but are not exactly prancing horses.  More common are tools, and major advances seem to have taken place in Africa, long before they appear in Europe at around the same time as artistic impressions.  Photographs of the engraved ochre objects bear strong resemblance to recent “doodles” by hunter gatherers and even runestones or tally sticks.  It is certainly a case of “Who knows?”, until more finds come to light.


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